1. Field of the Invention
The present invention relates to an apparatus for determining a bifurcate portion of an artery which is present under skin of a living subject, and an apparatus for detecting a pulse wave from a pulse-wave-detect portion of the artery that is determined based on the bifurcate portion and obtaining accurate pulse-wave-propagation-relating information based on the pulse wave.
2. Discussion of Related Art
As physical information relating to circulatory organ of a living subject, there is known a pulse-wave propagation time in which a pulse wave propagates between two predetermined portions of an artery, and a pulse-wave propagation velocity which is calculated based on the pulse-wave propagation time and the distance between the two predetermined portions of the artery. Hereinafter, the pulse-wave propagation time and the pulse-wave propagation velocity will be referred to as the pulse-wave-propagation-relating information or PWP-relating information. The PWP-relating information reflects the blood pressure (BP) of the subject. Therefore, it has been practiced to monitor the PWP-relating information and, when a change thereof is detected, start an automatic BP measurement on the subject. It is also practiced to obtain, each time the heart of the subject beats, a piece of PWP-relating information and estimate, based on the obtained information, a BP value of the subject. In addition, since the PWP-relating information reflects the flexibility of arteries of the subject, the information is utilized to measure the degree of arteriosclerosis, and/or the degree of peripheral resistance, of the subject.
Meanwhile, a pulse-wave propagation time is obtained by detecting respective heartbeat-synchronous signals from two portions of an artery of a living subject and subtracting a first time at which a first heartbeat-synchronous signal from a first portion of the artery from a second time at which a second heartbeat-synchronous signal from a second portion of the artery; and a pulse-wave propagation velocity is obtained by dividing the distance between the first and second portions of the artery by the pulse-wave propagation time. Two sensors are used to detect the first and second heartbeat-synchronous signals from the first and second portions of the artery, respectively. At least one of the two sensors is provided by a pressure-pulse-wave sensor which presses an appropriate portion of the artery via skin of the subject and detects, from the pressed portion, a pressure pulse wave which is produced in synchronism with the heartbeat of the subject. In this case, usually, a characteristic point of the waveform of the thus detected pressure pulse wave, such as a rising point (i.e., a minimum-value point), a maximum-slope point, a maximum-value point, or a well-known xe2x80x9cnotchxe2x80x9d point, is employed as a reference point to obtain a piece of PWB-relating information.
However, it is difficult for a living subject or a medical staff (e.g., a doctor or a nurse) to iteratively press the pressure-pulse-wave sensor against substantially the same portion of the artery to detect a pressure pulse wave in respective measurements at different times. Therefore, the distance between the two portions of the artery that is used to measure the pulse-wave propagation time or velocity may change between the different measurements. Accordingly, the thus measured propagation times or velocities cannot enjoy a satisfactory accuracy. Thus, it is difficult to compare the propagation times or velocities measured at different times, with each other, or detect a time-wise change of the propagation times or velocities.
In this background, one may possibly think to use a bone near an artery, as a reference point, and press the pressure-pulse-wave sensor at a measure point distant by a predetermined distance from the reference point. For example, a base portion of a costa of each living subject may be used as a reference point, and a predetermined distance from the reference point to a measure point above a carotid artery may be recorded or stored for the each subject. In this case, in each of respective measurements at different times, the sensor is pressed at the measure point above the carotid artery that is distant by the predetermined distance from the reference point. However, generally, a living person is mechanically soft and it is difficult to fix his or her joints. Thus, it is difficult to fix his or her head or neck in the same posture in each measurement. Therefore, even if the sensor may be accurately pressed at the measure point above the carotid artery distant by the predetermined distance from the base portion of the costa, the sensor may not be accurately pressed against substantially the same portion of the artery in every measurement.
It is therefore an object of the present invention to provide a pulse-wave-propagation-relating-information obtaining apparatus and an arterial-bifurcate-portion determining apparatus each of which assures accurate measurement of pulse-wave-propagation-relating information.
To achieve the above object, the present inventors have carried out an extensive study, and found the fact that when a bifurcate portion of an artery, or a portion of the artery distant by a predetermined distance from the bifurcate portion is used as a pulse-wave-detect portion, accurate pulse waves can be detected because those pulse waves are free of the conventionally encountered problem that subject""s head or neck cannot be fixed to the same posture in every measurement because of his or her mechanical softness and/of the difficulty of fixing of his or her joints. The present invention has been developed based on this finding.
The present invention provides a pulse-wave-propagation (PWP) relating information obtaining apparatus and an arterial-bifurcate-portion determining apparatus which have one or more of the following technical features that are described below in respective paragraphs given parenthesized sequential numbers (1) to (8). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter feature. However, the following technical features and the appropriate combinations thereof are just examples to which the present invention is by no means limited.
(1) According to a first feature of the present invention, there is provided an apparatus for obtaining a pulse-wave-propagation-relating information of a living subject, comprising a bifurcate-portion determining device which determines a bifurcate portion of an artery of the subject that is present under a skin of the subject; a pulse-wave-detect-portion determining means for determining, based on the determined bifurcate portion of the artery, a pulse-wave-detect portion of the artery from which a pulse wave is detected; a reference-point determining means for determining a reference point on the pulse wave detected from the determined pulse-wave-detect portion of the artery; and a pulse-wave-propagation-relating-information obtaining means for obtaining the pulse-wave-propagation-relating information of the subject based on the determined reference point of the pulse wave.
In the present PWP-relating-information obtaining apparatus, the pulse-wave-detect-portion determining means determines, based on the bifurcate portion of the artery determined by the bifurcate-portion determining device, a pulse-wave-detect portion of the artery, and the reference-point determining means determines a reference point on the pulse wave detected from the determined pulse-wave-detect portion of the artery. Then, the PWP-relating-information obtaining means obtains the PWP-relating information of the subject based on the determined reference point of the pulse wave. Thus, the pulse-wave-detect portion is determined based on the bifurcate portion of the artery itself, and the PWP-relating information is obtained based on the reference point of the pulse wave detected from the pulse-wave-detect portion. The thus determined pulse-wave-detect portion of the artery is not changed by the change of posture of the living subject. Therefore, accurate PWP-relating information can be obtained. This advantage is amplified under such measurement conditions that a distance between the pulse-wave-detect portion and another portion of the artery is short, i.e., a time in which the pulse wave propagates between the two portions of the artery is short, for example, in such a case where PWB-relating information is obtained from between the reference point of the pulse wave detected from a carotid artery and a reference point on the waveform of a photocardiogram or an electrocardiogram.
(2) According to a second feature of the present invention that includes the first feature (1), the bifurcate-portion determining device comprises a pressure-pulse-wave sensor including a plurality of pressure-sensing elements which are arranged at a first regular interval of distance in an arrangement direction over a distance longer than a diameter of the artery, and each of which produces a pulse-wave signal representing a pressure pulse wave propagated thereto from the artery; a distribution-curve obtaining means for obtaining a plurality of distribution curves each of which represents a relationship between respective positions of the pressure-sensing elements in the arrangement direction and respective amplitudes or respective minimum magnitudes of the respective pressure pulse waves represented by the respective pulse-wave signals produced by the pressure-sensing elements which are located at a corresponding one of a plurality of curve-obtain positions which are arranged at a second regular interval of distance in a predetermined range in a lengthwise direction of the artery and at each of which the pressure-sensing elements arranged in the arrangement direction traverse the artery; and a bifurcate-portion determining means for determining, based on the distribution curves respectively corresponding to the curve-obtain positions, the bifurcate portion of the artery in the predetermined range in the lengthwise direction of the artery. In the present PWP-relating information obtaining apparatus, the distribution-curve obtaining means obtains a plurality of distribution curves each of which represents a relationship between respective positions of the pressure-sensing elements in the arrangement direction and respective amplitudes or respective minimum magnitudes of the respective pressure pulse waves represented by the respective pulse-wave signals produced by the pressure-sensing elements which are located at a corresponding one of a plurality of curve-obtain positions which are arranged at a second regular interval of distance in a predetermined range in a lengthwise direction of the artery and at each of which the pressure-sensing elements arranged in the arrangement direction traverse the artery; and the bifurcate-portion determining means determines, based on the distribution curves respectively corresponding to the curve-obtain positions, the bifurcate portion of the artery. Thus, an accurate bifurcate-portion can be determined, and accordingly an accurate pulse-wave-detect portion can be determined, so that accurate PWP-relating information can be obtained.
(3) According to a third feature of the present invention that includes the second feature (2), the bifurcate-portion determining means determines the bifurcate portion of the artery based on at least one second interval between the two curve-obtain positions one of which corresponds to the distribution curve having a single peak and the other of which corresponds to the distribution curve having two peaks. In the present PWP-relating information obtaining apparatus, the bifurcate-portion determining means determines the bifurcate portion based on one or more intervals between two curve-obtain positions one of which corresponds to a distribution curve having a single peak and the other of which corresponds to another distribution curve having two peaks, more preferably, within the interval or intervals. Therefore, an accurate bifurcate portion can be determined, and accordingly accurate PWP-relating information can be obtained.
(4) According to a fourth feature of the present invention that includes any one of the first to third features (1) to (3), the pulse-wave-detect-portion determining means determines, as the pulse-wave-detect portion of the artery, a portion of the artery that is distant, by a predetermined distance, downstream of the determined bifurcate portion of the artery in a direction in which blood flows in the artery. In the present PWP-relating information obtaining apparatus, a portion of the artery that is not adversely influenced by turbulent flows produced in the vicinity of the bifurcate portion can be determined as the pulse-wave-detect portion of the artery, and an a good pulse wave having an accurate waveform can be detected from the pulse-wave-detect portion of the artery. Thus, an accurate reference point can be determined on the waveform of pressure pulse wave. In addition, since a distance by, or a time in, which the pressure pulse wave propagates is increased, the accuracy with which pulse-wave-propagation velocity is calculated based on time difference is increased.
(5) According to a fifth feature of the present invention that includes any one of the thirst to fourth features (1) to (4), the pulse-wave-propagation-relating-information obtaining means obtains the pulse-wave-propagation-relating information of the subject based on a time difference between a reference point on a waveform of an electrocardiogram or a phonocardiogram and the determined reference point of a pressure pulse wave as the pulse wave detected from the determined pulse-wave-detect portion of a carotid artery as the artery. In the present PWP-relating information obtaining apparatus, a pressure pulse wave having substantially the same waveform as that of an aortic pressure pulse wave can be detected from the carotid artery, and a time difference can be measured based on a reference point determined on the accurate waveform of the pressure pulse wave detected from the pulse-wave-detect portion of the carotid artery. Therefore, though the carotid artery is not so distant from the heart of the subject, accurate PBP-relating information of the subject can be obtained.
(6) According to a sixth feature of the present invention that includes the second feature (2), the pressure-pulse-wave sensor includes the pressure-sensing elements which are arranged in a matrix such that the elements are arranged at a regular interval of distance along a plurality of first straight lines parallel to each other and at a regular interval of distance along a plurality of second straight lines which are parallel to each other and are perpendicular to the first straight lines. The present PWP-relating-information obtaining apparatus need not employ a drive device for moving the pressure-pulse-wave sensor in the directions along the artery, in contrast to the case where the pressure pulse wave sensor includes only a single array of pressure-sensing elements which is adapted to intersect the artery when the sensor is applied to the subject.
(7) According to a seventh feature of the present invention, there is provided an apparatus for determining a bifurcate portion of an artery of a living subject that is present under a skin of the subject, comprising a pressure-pulse-wave sensor including a plurality of pressure-sensing elements which are arranged at a first regular interval of distance in an arrangement direction over a distance longer than a diameter of the artery, and each of which produces a pulse-wave signal representing a pressure pulse wave propagated thereto from the artery; a distribution-curve obtaining means for obtaining a plurality of distribution curves each of which represents a relationship between respective positions of the pressure-sensing elements in the arrangement direction and respective amplitudes or respective minimum magnitudes of the respective pressure pulse waves represented by the respective pulse-wave signals produced by the pressure-sensing elements which are located at a corresponding one of a plurality of curve-obtain positions which are arranged at a second regular interval of distance in a predetermined range in a lengthwise direction of the artery and at each of which the pressure-sensing elements arranged in the arrangement direction traverse the artery; and a bifurcate-portion determining means for determining, based on the distribution curves respectively corresponding to the curve-obtain positions, the bifurcate portion of the artery in the predetermined range in the lengthwise direction of the artery.
In the present bifurcate-portion determining apparatus, the distribution-curve obtaining means obtains a plurality of distribution curves each of which represents a relationship between respective positions of the pressure-sensing elements in the arrangement direction and respective amplitudes or respective minimum magnitudes of the respective pressure pulse waves represented by the respective pulse-wave signals produced by the pressure-sensing elements which are located at a corresponding one of a plurality of curve-obtain positions which are arranged at a second regular interval of distance in a predetermined range in a lengthwise direction of the artery and at each of which the pressure-sensing elements arranged in the arrangement direction traverse the artery, and the bifurcate-portion determining means determines, based on the distribution curves respectively corresponding to the curve-obtain positions, the bifurcate portion of the artery in the predetermined range in the lengthwise direction of the artery. Therefore, the present apparatus can accurately determine the bifurcate portion of the artery. In addition, since a pulse-wave-detect portion of the artery can be accurately determined based on the accurately determined bifurcate portion, accurate PWP-relating information can be obtained based on a pulse wave detected from the accurately determined pulse-wave-detect portion.
(8) According to an eighth feature of the present invention that includes the seventh feature (7), the bifurcate-portion determining means determines the bifurcate portion of the artery based on at least one second interval between the two curve-obtain positions one of which corresponds to the distribution curve having a single peak and the other of which corresponds to the distribution curve having two peaks. In the present bifurcate-portion determining apparatus, the bifurcate-portion determining means determines the bifurcate portion based on one or more intervals between two curve-obtain positions one of which corresponds to a distribution curve having a single peak and the other of which corresponds to another distribution curve having two peaks, more preferably, within the interval or intervals. Therefore, an accurate bifurcate portion can be determined, and accordingly an accurate pulse-wave-detect portion of the artery can be determined based on the bifurcate portion. Consequently accurate PWP-relating information can be obtained.